1. Field of the Invention
This invention relates to a control method of phase transition of fractal-coupled structures, fractal-coupled structures, ferromagnetic fractal-coupled structures, information processing method, information storage method, information storage medium, information processing device and information storage device, which, in particular, are based on a new principle.
2. Background Art
Materials exhibiting ferromagnetism are widely used as storage mediums, and support present technologies. Not only bulk magnetic materials but also those variously designed in layered structures are used, and they are employed in, for example, magneto-optical discs (MO discs). There are also vigorous researches and developments toward future magnetic materials, and in recent years, powder magnetic materials, i.e. magnetic particles, have been remarked ((1) J. M. L. Billas, A. Chatelain, W. A. de Heer, Science, 265, 1682(1994), (2) D. Gatteschi, A. Caneschi, L. Pardi and R. Sessoli, Science, 265, 1054(1994)).
On the other hand, inherent non-linearity is indispensable as a physical system assuming information processing. Although linearly responsive ones are also used as devices, they cannot be active devices. As devices used conventionally, there are electronic devices using materials that exhibit non-linear responses to a certain extent. For example, two-terminal devices exhibiting differential negative resistance are an example of those having non-linearity in current-voltage characteristics. Of course, three-terminal MOS-FETs also support the present techniques. Then, by coupling these electronic devices having non-linearity with linear electronic circuits and thereby building an information processing apparatus having non-linearity, any desired calculation can be performed.
However, difficulties by high integration have become issues with such electronic circuits. Heating is one of such problems. Heating caused by inherent electric resistance is indispensable for creating non-linearity of an electronic device, indispensable for executing information processing, and therefore essential.
In order to overcome the difficulty, trials have been made to decrease devices by enhancing non-linearity of components elements. Progress of this scheme necessarily leads to the demand for component devices having as strong non-linearity as exhibiting a chaos. When a classical system exhibiting a chaos is quantized, what characterizes the behaviors of the quantum system is a quantum chaos.
On the other hand, as a component device is minimized more and more, electrons confined in the device will behave as quantum-mechanic particles. Therefore, from this viewpoint, hopes are placed on component devices exhibiting a chaos.
For application of a solid material to electronic or optical devices, physical properties of the material may restrict its applications. For example, in case of using a semiconductor material in a light emitting device, it will be usable in a device of an emission wavelength corresponding to the band gap of the material, but some consideration will be necessary for changing the emission wavelength. Regarding physical properties related to semiconductor bands, controls by superlattices have been realized. More specifically, by changing the period of a superlattice, the band width of its subband can be controlled to design an emission wavelength.
Targeting on controlling many-electron-state structures by material designs, the Inventors proposed many-body effect engineering by quantum dot coupled structures and has continued theoretical analyses ((3) U.S. Pat. No. 5,430,309; (4) U.S. Pat. No. 5,663,571; (5) U.S. Pat. No. 5,719,407; (6) U.S. Pat. No. 5,828,090; (7) U.S. Pat. No. 5,831,294; (8) J. Appl. Phys. 76, 2833(1994); (9) Phys. Rev. B51, 10714(1995); (10) Phys. Rev. B51, 11136(1995); (11) J. Appl. Phys. 77, 5509(1995); (12) Phys. Rev. B53, 6963(1996); (13) Phys. Rev. B53, 10141(1996); (14) Appl. Phys. Lett. 68, 2657(1996); (15) J. Appl. Phys. 80, 3893(1996); (16) J. Phys. Soc. Jpn. 65, 3952(1996); (17) Jpn. J. Appl. Phys. 36, 638(1997); (18) J. Phys. Soc. Jpn. 66, 425(1997); (19) Jpn. J. Appl. Phys. 81, 2693(1997); (20) Physica (Amsterdam) 229B, 146(1997); (21) Physica (Amsterdam) 237A, 220(1997); (22) Surf. Sci. 375, 403(1997); (23) Physica (Amsterdam) 240B, 116(1997); (24) Physica (Amsterdam) 240B, 128(1997); (25) Physica (Amsterdam) 1E, 226(1997); (26) Phys. Rev. Lett. 80, 572(1998); (27) Jpn. J. Appl. Phys. 37, 863(1998); (28) Physica (Amsterdam) 245B, 311(1998); (29) Physica (Amsterdam) 235B, 96(1998); (30) Phys. Rev. B59, 4952(1999); (31) Surf. Sci. 432, 1(1999); (32) International Journal of Modern Physics B. Vol. 13, No. 21, 22, pp. 2689–2703, 1999). For example, realization of various correlated electronic systems is expected by adjusting a tunneling phenomenon between quantum dots and interaction between electrons. Let the tunneling transfer between adjacent quantum dots be written as t. Then, if quantum dots are aligned in form of a square lattice, the bandwidth of one electron state is Teff=4t. If quantum dots form a one-dimensional chain, the band width of one electron state is Teff=2t. In case of a three-dimensional quantum dot array, Teff=6t. That is, if D is the dimension of a quantum dot array, the band width of one electron state has been Teff=2Dt.
When a magnetic material is used in a storage medium, it may be necessary to heat it high above the temperature of its ferromagnetic phase transition for, for example, erasure of storage, and this is a constraint on its applications. Therefore, if the ferromagnetic phase transition temperature can be controlled by changing some parameter without heating, the possibility of various technical applications will be extended.
On the other hand, regarding devices using a quantum chaos, it is known that degeneracy in density of states occurs due to the self-similarity in a quantum system having a fractal structure. Although this itself is useful, the quantum chaos the system exhibits is defined by GOE (Gaussian orthogonal ensemble) distribution, it is relatively weak in terms of quantum chaos. Thus there is a demand for realization of a stronger quantum chaos.
Further, consideration is made about half-filled (one electron per each quantum dot) Mott transition (also called Mott-Hubbard transition or Mott metal-insulator transition). Let the effective electrons interaction within a quantum dot be written as Ueff, then the Hubbard gap on the part of the Mott insulator is substantially described as Δ=Ueff−Teff, and the Mott transition can be controlled by changing Ueff or t. As already proposed, the Mott-Hubbard transition can be controlled by adjusting Ueff or t, using a field effect, it is applicable to field effect devices (References (7), (8), (13) and (16) introduced above). However, if the Mott-Hubbard transition can be controlled by controlling a parameter other than the field effect, it is more advantageous for increasing the possibility of its applications.
It is therefore an object of the invention to provide a method of controlling phase transition of a fractal-coupled structure, which can control phase transition such as ferromagnetic phase transition without the need for heating, unlike the conventional techniques.
Another object of the invention is to provide a fractal-coupled structure and a ferromagnetic fractal-coupled structure in which the above-indicated control method can be used.
A further object of the invention is to provide an information processing method, information storage method, information storage medium, information processing device and information storage device that use the above-mentioned control method or fractal-coupled structure.